Conventional apparatuses for powering electric motors are known, for example, power combos or other powering devices used for hard disks in computers. A device for powering an electric motor basically comprises a device for driving the electric motor, and a control device suitable for regulating the supply voltage, which comes from a voltage generator outside the powering device, that is input to the driving device, and preferably suitable for controlling the electronic components belonging to the driving device. The regulation of the input supply voltage of the driving device is obtained by suitably controlling a device, preferably constituted by a MOSFET transistor and an intrinsic diode, that is inserted into the supply line that connects the voltage generator to the device for driving the electric motor.
A conventional power combo 1 is shown in FIG. 1. A supply voltage VCV coming from a voltage generator 2 is in input to the power combo 1. The power combo comprises a first driving device 3 for driving a spindle motor 4 that is outside the power combo 1, and a second driving device 5 for driving a voice coil motor 6, which is also outside the power combo 1. Still inside the power combo 1, a supply line 7 is provided between the voltage generator 2 and the driving devices 3 and 5. A MOSFET device 8 is inserted in this supply line 7, preferably an ISO-FET device comprising a MOS transistor M1 and an intrinsic diode D1. The driving devices 3 and 5 and the transistor M1 are controlled by a single control device 10 inside the power combo 1 and powered by the supply voltage VCV. A serial interface 20 having external input signals, such as clock and data signals, and being suitable for sending command signals to the control device 10, and a block 30 comprising further voltage regulators suitable for sending out regulated voltage signals are typically also provided inside the power combo 1.
During the operation of an electric motor, such as a voice coil or a spindle motor, operating conditions can occur such as to produce an increasing of the voltage between a circuit node VM, located downstream from the MOSFET device 8 and upstream from the driving devices 3 and 5, and ground. This is due to the current, which instead of flowing from the voltage generator 2 to the devices 3 and 5, flows in the opposite direction through the MOSFET device 8.
Such a situation is encountered, for example, when a voltage lower than the voltage of BEMF (Back Electromotive Force) generated by the motor itself is applied to the electric motor, in the case in which it is in rotation.
Another similar situation occurs when the driving devices 3 and 5, which are formed by MOS transistors, are driven in high impedance. The current that flows in the electric motor recirculates towards the voltage generator 2 through the intrinsic diodes of the MOS transistors of the driving devices 3 and 5.
In both of these situations, if the voltage generator 2 presents a component of high impedance towards ground it cannot absorb the current that flows from the driving devices 3 and 5 towards it. The current generates an over-voltage that can reach more or less high values according to the filter capacitances, that is the capacitance C1 connected between the supply voltage and ground and the capacitance C2 connected between the node VM and ground, which are used to filter the supply voltage VCV. The capacitances C1 and C2 are outside the circuit block in which the elements belonging to the apparatus 1 are integrated, that is the circuit elements 3, 5, 8, 10, 20 and 30. The over-voltage can reach high values such that it exceeds the voltage needed for the operating of the driving devices 3 and 5.
Methods for limiting the over-voltage are known that are based on the use of high value capacitance, for example increasing the value of the capacitances C1 and C2 by a few microfarads to 50 or 100 microfarads, or on the use of, also in combination with the high value capacitance, voltage suppressors, such as the Transil diode Dz of FIG. 1 positioned between the node VM and ground, that are suitable for limiting the supply voltage.
However, the known solutions are very expensive and bulky and are not in line with the current technological trends in the research of devices that are less and less bulky and not expensive.